Hydronic coolant overflow

Woke up this morning to find coolant dripping from the ceiling, and upon investigation I discovered the hydronic heating loop overflow tank (plastic container at top of pic 1) spraying out inside the pilothouse steering station.

After much cursing and rummaging for containers to collect the overflow here and the dripping elsewhere (~3 gal) the overflow still wasn't slowing down at all. I did notice the color change from green to clear, so I started to suspect mixing between the coolant loop and fresh water circuit. After setting up a shop vac to give me my hands back I shut off the fresh water pump, bled the water pressure out, and confirmed the coolant expansion tank (grey tank with silver cap at bottom of pic 1) stopped overflowing.

The two fluid loops meet in the water heater (Raritan 172011), so I drained it and can now run the water pump briefly to wash dishes, etc, without any coolant expansion tank overflow.

Coolant circuit is something like this:
1. Pump
2. Webasto furnace
3. Standalone heat exchanger (exchange with engine coolant, pic 2)
4. Water heater
5. Several cabin fan radiators
6. Expansion tank
7. Return to pump

Standalone heat exchanger looks to be leaking at heater coolant input due to failed hose clamp, but I don't see how that's related to the overflow issue.

So my question is whether this is a common failure mode in heat exchanger water heaters? Poor zinc maintenance on my (and previous owner's) part?

I've had one fail before at the heat exchanger nipple causing the tank to leak into the bilge. I don't see any other evidence of leaks on this one, so it must be internal to the tank where coolant and fresh water are mixing. I'm guessing this one is a lost cause and needs to be completely replaced.

Welcome aboard. Sorry I can’t help as I have no experience with those, but I did want to say hello and welcome.

I was able to isolate water heater from the coolant loop, but now I'm wondering if I need to worry about ethylene glycol contamination in the fresh water. Is there a way to easily test for that?

Can’t say if it’s common or not but the danger of getting ethylene glycol in the hot water is why I use propylene glycol based coolant in my hydronic system. It’s really not that much more expensive so I use it in my engine also as I have a separate heat exchanger for the engine to hydronic system. So two heat exchangers would have to leak before I got engine coolant in my drinking water but just seems like cheap insurance and I only have to carry one type of coolant. Since your water pressure is higher than the hydronic system pressure you may have flushed everything away from your drinking water but ethylene glycol is hard on the kidneys so personally I’d isolate and not use the hot water until I could replace the heater and flush the lines. Never heard of a test for small quantities of EG (lots of testers for freeze protection) there are blood tests Veterinarians use them to check for poisoning.

I have had two heat exchangers fail, with coolant crossing from one to the other. I'm told this is rare, but I've had two...

This is why you never run ethylene glycol in a hydronic system which has a heat exchanger to domestic water. Most hydronic manufacturers do not mean the pink antifreeze that you use for winterization - there are specific PP glycol fluids sold for this purpose.

Ethylene glycol poisoning is not to be trifled with, very bad.

How do you fill the system. Strictly thru the overflow tank, or is there a water line to the system?

Bigger hydronic systems usually have a makeup water valve to ensure the system doesn't run low and make steam instead of hot water. Usually there's a pressure valve. My current one maintains 15psi. There's also a pressure relief valve in case the furnace makes steam.

A leak in the system or a failure of the pressure relief valve causes the makeup water pressure valve to add water. If there is a makeup water connection and if there is a pressure relief valve venting to the overflow that has failed, that would explain the change in coolant color.

I agree with using propylene glycol rather than ethylene glycol in the heat loops. Are you even sure you have ethylene glycol currently? Prop glycol has been common in heating system for quite a while now.



That said, it's unlikely your water is contaminated for the exact reason your heating system flooded - the water system is at a higher pressure, so fresh water flows into the heating system, not the other way around. Regardless, once the heating system has been isolated from the HW tank, I'd run the hot water for an extended time to flush it out.

The problem with an exchange of fluids is you cannot assume it is one way. One side heats up or gets pressurized and fluid flows one way. Then that side cools or the other side gets pressurized, fluid flows the other way. That is what happened in my failures, the hydronic mainly pushed into the engine but based on color the engine was sometimes pushing into the hydronic. They are hot (and pressurized) at different times. In his example, if the water system is depressurized when the hydronic is hot, it's going to get some coolant in it.

DDW said:

The problem with an exchange of fluids is you cannot assume it is one way. One side heats up or gets pressurized and fluid flows one way. Then that side cools or the other side gets pressurized, fluid flows the other way. That is what happened in my failures, the hydronic mainly pushed into the engine but based on color the engine was sometimes pushing into the hydronic. They are hot (and pressurized) at different times. In his example, if the water system is depressurized when the hydronic is hot, it's going to get some coolant in it.

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Why is the water system depressurized when the hydronic is hot?

Turned the pump off? The OP already said he did that to stop the bleeding....

Even if it might be an unlikely condition, I would not risk antifreeze poisoning. Fix it, flush the water system with fresh. Fortunately ethylene glycol dilutes readily in water.

Lepke said:

How do you fill the system. Strictly thru the overflow tank, or is there a water line to the system?

Click to expand...

The only fill I've seen is via the expansion tank (silver cap on the grey cylinder on the lower left of the first picture). I haven't seen a water line interface to the hydronic loop.

twistedtree said:

Are you even sure you have ethylene glycol currently? Prop glycol has been common in heating system for quite a while now.

Click to expand...

I haven't refilled the system since I purchased the boat, so I'm not as sure as I could be, but there is a jug of ethylene glycol antifreeze next to the expansion tank where the loop gets filled.

DDW said:

Turned the pump off? The OP already said he did that to stop the bleeding....

Even if it might be an unlikely condition, I would not risk antifreeze poisoning. Fix it, flush the water system with fresh. Fortunately ethylene glycol dilutes readily in water.

Click to expand...

After isolating it from the hydronic loop, I flushed the hot water tank several times and ran a lot of water through it. Ordered some test kits that all showed we are clean.

Now looking at a hurricane heater upgrade I was already planning and wondering if I even need to replace this hot water tank.

The water heater can be expected to last about 10 years. The coolant hoses to the engine should be replaced at this time as well.

fkerekes said:

After isolating it from the hydronic loop, I flushed the hot water tank several times and ran a lot of water through it. Ordered some test kits that all showed we are clean.

Now looking at a hurricane heater upgrade I was already planning and wondering if I even need to replace this hot water tank.

Click to expand...

Looking back at your original post, I'm a bit unclear where the hydronic loop and your fresh water meet. If I follow your earlier description, it's in the hot water heater/tank, and the stand along heat exchanger is where the hydronic loop meets the engine aux heat loop.


Is that right? Because if so, the leak is in your hot water tank. No?